The present invention relates to new propylene polymer compositions having improved impact strength and excellent optical properties.
As is known, the isotactic polypropylene is endowed with an exceptional combination of excellent properties which render it suitable for a very great number of uses; however, it exhibits the drawback of possessing an insufficient impact strength. Attempts were made to obviate such drawback, either by properly modifying the synthesis process or by blending with rubbers.
The modifications to the synthesis process essentially consist in introducing into the propylene stereoregular homopolymerization process one or more copolymerization steps. In particular, it is known that for certain applications it is useful to decrease the crystallinity of the propylene homopolymer by copolymerization of the propylene with small quantities of ethylene and/or xcex1-olefins such as 1-butene, 1-pentene and 1-hexene. In this manner one obtains the so called random crystalline propylene copolymers which, when compared to the homopolymer, are essentially characterized by better flexibility and transparency.
These materials can be used in many application sectors, such as, for example irrigation pipes, pipes for transporting drinking water and other liquid food, heating equipments, single layer bottles (for detergents), multilayer bottles (for beverages and perfumes), single layer or multilayer film for various packaging and rigid food containers.
Propylene random copolymers, however, although they have good transparency, do not offer, especially at low temperatures, sufficiently better impact resistance than the homopolymer which can be satisfactory used for the applications listed above.
It has been known for a long time that the impact resistance of polypropylene can be improved by adding an adequate quantity of elastomeric propylene-ethylene copolymer to the homopolymers by mechanical blending or sequential polymerization. However, this improvement is obtained at the expenses of the transparency of the material.
To avoid this inconvenient, U.S. Pat. No. 4,634,740 suggests the blending of the polypropylene, in the molten state, with propylene-ethylene copolymers obtained with specific catalysts, and having an ethylene content ranging from 70 to 85% by weight. However, said compositions present transparency values (Haze) substantially comparable to those of the propylene homopolymer. Said patent, therefore, does not teach how to obtain compositions having good transparency.
In order to overcome the above mentioned disadvantages the Applicant has previously produced transparent polypropylene compositions offering impact resistance at low temperatures, which can be prepared directly in polymerization (sequential copolymerization). Said compositions, which constitutes the subject of published patent application EP-A-373660, comprise both a crystalline random propylene copolymer, and an elastomeric copolymer of ethylene with propylene and/or C4-C8 xcex1-olefins containing from 20 to 70% by weight of ethylene. According to said patent application, the compositions mentioned above have a good balance of mechanical and chemical-physical properties due to the fact that the value of the content of ethylene of the elastomeric copolymer multiplied by the value of the ratio between the intrinsic viscosity (I.V.) of the elastomeric copolymer soluble in xylene at ambient temperature, and the one of the propylene random copolymer is comprised within a predetermined range.
Published European patent application EP-A-0557953, in the name of the Applicant, describes polyolefin compositions where one obtains a good balance of transparency, stiffness, and impact resistance even at low temperatures, by modifying a crystalline random copolymer of propylene with the proper quantities of a mechanical mixture comprising an elastomeric copolymer and one or more polymers chosen from LLDPE, LDPE and HDPE. In the U.S. Pat. No. 5,994,482 polypropylene alloys for fibers are described, containing two different propylene/ethylene copolymers.
New polypropylene compositions have now been found which have an optimum balance of transparency, stiffness and impact resistance even at low temperatures.
Thus the present invention provides propylene polymer compositions comprising (percent by weight):
A) from 70 to 90%, preferably from 75 to 85%, of a random copolymer of propylene with ethylene, containing from 1 to 6%, preferably from 1.5 to 4%, of ethylene, having a content of fraction insoluble in xylene at room temperature (about 23xc2x0 C.) of not less than 93%, preferably not less than 94%;
B) from 10% to 30%, preferably from 15% to 25%, of a copolymer of propylene with ethylene, containing from 8 to 18%, preferably from 10 to 18%, of ethylene;
wherein the ratio (B)/C2B of the percent by weight of (B), with respect to the total weight of (A) and (B), to the percent by weight of ethylene in (B), with respect to the total weight of (B), represented in the above formula by C2B, is 2.5 or lower, preferably 2 or lower. The lower limit of said ratio is preferably 0.5.
The term xe2x80x9ccopolymerxe2x80x9d includes polymers containing more than one kind of comonomers. In particular, in addition to ethylene, also one or more C4-C10 xcex1-olefin(s) can be present, provided that components A) and B) remain miscible. In fact a preferred feature of the compositions of the present invention is that only one polymer phase (without substantial presence of other immiscible polymer phases) is detected by microscopy analysis. Other preferred features for the compositions of the present invention are:
MFR L (Melt Flow Rate according to ASTM 1238, condition L, i.e. 230xc2x0 C. and 2.16 kg load) from 0.5 to 50, in particular from 0.5 to 10 g/10 min.; more preferably, in particular for the preparation of bottles and containers (extrusion blow molding), from 0.5 to 2.5, most preferably from 1 to 2.5, for example from 1 to 2 g/10 min.;
Polydispersity Index (PI): 5 or less, more preferably 4 or less;
Intrinsic Viscosity [xcex7] of the fraction (of the overall composition) insoluble in xylene at room temperature: from 1.5 to 3, more preferably from 2 to 2.5 dl/g;
Intrinsic Viscosity [xcex7] of the fraction (of the overall composition) soluble in xylene at room temperature: from 1 to 4.5, more preferably from 1.5 to 4 dl/g;
The compositions of the present invention present at least one melt peak, determined by way of DSC (Differential Scanning Calorimetry), at a temperature higher than 140-145xc2x0 C. Moreover, the compositions of the present invention preferably have:
a Flexural Modulus of 600 MPa or higher;
Haze (measured on samples containing 2000 ppm DBS, i.e. di-benzylidenesorbitol): 12% or less on 1 mm plaques; 10% or less on 50 xcexcm films;
Gloss (on films): 45% or more;
fraction extractable in hexane (FDA 177, 1520): less than 5%, more preferably less than 3% by weight;
fraction soluble in xylene at room temperature: less than 25%, more preferably less than 20%.
The said C4-C10 xcex1-olefins, that may be present as comonomers in the compositions of the present invention, are represented by the formula CH2xe2x95x90CHR, wherein R is an alkyl radical, linear or branched, with 2-8 carbon atoms or an aryl (in particular phenyl) radical. Examples of said C4-C10 xcex1-olefins are 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene. Particularly preferred is 1-butene.
The compositions of the present invention can be prepared by sequential polymerization in at least two polymerization steps. Such polymerization is carried out in the presence of stereospecific Ziegler-Natta catalysts. An essential component of said catalysts is a solid catalyst component comprising a titanium compound having at least one titanium-halogen bond, and an electron-donor compound, both supported on a magnesium halide in active form. Another essential component (co-catalyst) is an organoaluminum compound, such as an aluminum alkyl compound.
An external donor is optionally added.
The catalysts generally used in the process of the invention are capable of producing polypropylene with an Isotacticity Index greater than 90%, preferably greater than 95%. Moreover, said catalysts must have a sensitivity to molecular weight regulators particularly hydrogen) high enough to produce polypropylene having MFR values from less than 1 g/10 min. to 100 g/10 min. or more.
Catalysts having the above mentioned characteristics are well known in the patent literature; particularly advantageous are the catalysts described in U.S. Pat. No. 4,399,054 and European patent 45977. Other examples can be found in U.S. Pat. No. 4,472,524.
The solid catalyst components used in said catalysts comprise, as electron-donors (internal donors), compounds selected from the group consisting of ethers, ketones, lactones, compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic acids. Particularly suitable electron-donor compounds are 1,3-diethers of formula: 
wherein RI and RII are the same or different and are C1-C18 alkyl, C3-C18 cycloalkyl or C7-C18 aryl radicals; RIII and RIV are the same or different and are C1-C4 alkyl radicals; or are the 1,3-diethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6, or 7 carbon atoms, or of 5-n or 6-nxe2x80x2 carbon atoms, and respectively n nitrogen atoms and nxe2x80x2 heteroatoms selected from the group consisting of N, O, S and Si, where n is 1 or 2 and nxe2x80x2 is 1, 2, or 3, said structure containing two or three unsaturations (cyclopolyenic structure), and optionally being condensed with other cyclic structures, or substituted with one or more substituents selected from the group consisting of linear or branched alkyl radicals; cycloalkyl, aryl, aralkyl, alkaryl radicals and halogens, or being condensed with other cyclic structures and substituted with one or more of the above mentioned substituents that can also be bonded to the condensed cyclic structures; one or more of the above mentioned alkyl, cycloalkyl, aryl, aralkyl, or alkaryl radicals and the condensed cyclic structures optionally containing one or more heteroatoms as substitutes for carbon or hydrogen atoms, or both.
Ethers of this type are described in published European patent applications 361493 and 728769.
Representative examples of said diethers are 2-methyl-2-isopropyl-1,3-dimethoxypropane; 2,2-diisobutyl-1,3-dimethoxypropane; 2-isopropyl-2-cyclopentyl-1,3-dimethoxypropane, 2-isopropyl-2-isoamyl-1,3-dimethoxypropane, and 9,9-bis (methoxymethyl) fluorene.
By using the said diethers, the preferred P.I. values mentioned above are obtained directly in polymerization.
Other suitable electron-donor compounds are phthalic acid esters, such as diisobutyl, dioctyl, diphenyl and benzylbutyl phthalate.
The preparation of the above mentioned catalyst components is carried out according to various methods.
For example, a MgCl2.nROH adduct (in particular in the form of spheroidal particles) wherein n is generally from 1 to 3 and ROH is ethanol, butanol or isobutanol, is reacted with an excess of TiCl4 containing the electron-donor compound. The reaction temperature is generally from 80 to 120xc2x0 C. The solid is then isolated and reacted once more with TiCl4, in the presence or absence of the electron-donor compound, after which it is separated and washed with aliquots of a hydrocarbon until all chlorine ions have disappeared.
In the solid catalyst component the titanium compound, expressed as Ti, is generally present in an amount from 0.5 to 10% by weight. The quantity of electron-donor compound which remains fixed on the solid catalyst component generally is 5 to 20% by moles with respect to the magnesium dihalide.
The titanium compounds which can be used for the preparation of the solid catalyst component are the halides and the halogen alcoholates of titanium. Titanium tetrachloride is the preferred compound.
The reactions described above result in the formation of a magnesium halide in active form. Other reactions are known in the literature, which cause the formation of magnesium halide in active form starting from magnesium compounds other than halides, such as magnesium carboxylates.
The active form of magnesium halide in the solid catalyst component can be recognized by the fact that in the X-ray spectrum of the catalyst component the maximum intensity reflection appearing in the spectrum of the nonactivated magnesium halide (having a surface area smaller than 3 m2/g) is no longer present, but in its place there is a halo with the maximum intensity shifted with respect to the position of the maximum intensity reflection of the nonactivated magnesium dihalide, or by the fact that the maximum intensity reflection shows a width at half-peak at least 30% greater than the one of the maximum intensity reflection which appears in the spectrum of the nonactivated magnesium halide. The most active forms are those where the above mentioned halo appears in the X-ray spectrum of the solid catalyst component.
Among magnesium halides, the magnesium chloride is preferred. In the case of the most active forms of magnesium chloride, the X-ray spectrum of the solid catalyst component shows a halo instead of the reflection which in the spectrum of the nonactivated chloride appears at 2.56 xc3x85.
The Al-alkyl compounds used as co-catalysts comprise the Al-trialkyls, such as Al-triethyl, Al-triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl compounds containing two or more Al atoms bonded to each other by way of O or N atoms, or SO4 or SO3 groups.
The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from 1 to 1000.
The electron-donor compounds that can be used as external donors include aromatic acid esters such as alkyl benzoates, and in particular silicon compounds containing at least one Si-OR bond, where R is a hydrocarbon radical.
Examples of silicon compounds are (tert-butyl)2 Si (OCH3)2, (cyclohexyl) (methyl) Si (OCH3)2, (phenyl)2 Si (OCH3)2 and (cyclopentyl)2 Si (OCH3)2. 1,3-Diethers having the formulae described above can also be used advantageously. If the internal donor is one of these diethers, the external donors can be omitted.
As previously said, the polymerization process can be carried out in at least two sequential steps, wherein components A) and B) are prepared in separate subsequent steps, operating in each step, except the first step, in the presence of the polymer formed and the catalyst used in the preceding step. The catalyst is generally added only in the first step, however its activity is such that it is still active for all the subsequent step(s).
Component A) is preferably prepared before component B).
The regulation of the molecular weight is carried out by using known regulators, hydrogen in particular.
By properly dosing the concentration of the molecular weight regulator in the relevant steps, the previously described MFR and [xcex7] values are obtained.
The whole polymerization process, which can be continuous or batch, is carried out following known techniques and operating in liquid phase, in the presence or not of inert diluent, or in gas phase, or by mixed liquid-gas techniques. It is preferable to carry out the propylene copolymerization step(s) for preparation of component A) in liquid propylene as diluent, and the other polymerization step(s) in gas phase. Generally there is no need for intermediate steps except for the degassing of unreacted monomers.
Reaction time, pressure and temperature relative to the two steps are not critical, however it is best if the temperature is from 20 to 100xc2x0 C. The pressure can be atmospheric or higher.
The catalysts can be pre-contacted with small amounts of olefins (prepolymerization). The compositions of the present invention can also be obtained by preparing separately the said components A) and B) by operating with the same catalysts and substantially under the same polymerization conditions as previously explained (except that a wholly sequential polymerization process will not be carried out, but the said components and fractions will be prepared in separate polymerization steps) and then mechanically blending said components and fractions in the molten or softened state. Conventional mixing apparatuses, like screw extruders, in particular twin screw extruders, can be used.
The compositions of the present invention can also contain additives commonly employed in the art, such as antioxidants, light stabilizers, heat stabilizers, nucleating agents, colorants and fillers.
In particular, the addition of nucleating agents brings about a considerable improvement in important physical-mechanical properties, such as Flexural Modulus, Heat Distortion
Temperature (HDT), tensile strength at yield and transparency.
Typical examples of nucleating agents are the p-tert.-butyl benzoate and the 1,3- and 2,4-dibenzylidenesorbitols.
The nucleating agents are preferably added to the compositions of the present invention in quantities ranging from 0.05 to 2% by weight, more preferably from 0.1 to 1% by weight with respect to the total weight.
The addition of inorganic fillers, such as talc, calcium carbonate and mineral fibers, also brings about an improvement to some mechanical properties, such as Flexural Modulus and HDT. Talc can also have a nucleating effect.
The compositions of the present invention are particularly suited for preparing bottles and containers with improved impact strength and excellent optical properties. Such bottles and containers can be monlayer or multilayer, optionally containing layers of other thermoplastic or elastoplastic polymers.
A preferred process for producing said bottles and containers is extrusion blow molding. Such process is well known in the relevant art, and comprises extrusion of a parison which is then blown with gas (air) to adhere to the inside surface of a mould, thereby obtaining the bottle or container.
The particulars are given in the following examples, which are given to illustrate, without limiting, the present invention.
Solid Catalyst Component
The solid catalyst component used in polymerization is a highly stereospecific Ziegler-Natta catalyst component supported on magnesium chloride, prepared as follows.
Into a 500 mL four-necked round flask, purged with nitrogen, 250 mL of TiCl4 were introduced at 0xc2x0 C. While stirring, 10.0 g of microspheroidal MgCl2*2.8C2H5OH (prepared according to the method described in ex.2 of U.S. Pat. No. 4,399,054 but operating at 3000 rpm instead of 10000 rpm) and 7.4 mmol of 9,9-bis(methoxymethyl)fluorene were added. The temperature was raised to 100xc2x0 C. and maintained for 120 min. Then, the stirring was discontinued, the solid product was allowed to settle and the supernatant liquid was siphoned off. Then 250 mL of fresh TiCl4 were added. The mixture was reacted at 120xc2x0 C. for 60 min and, then, the supernatant liquid was siphoned off. The solid was washed six times with anhydrous hexane (6xc3x97100 mL) at 60xc2x0 C. Finally, the solid was dried under vacuum and analyzed. The resulting solid catalyst component contained: Ti=3.5% by weight, 9,9-bis(methoxymethyl)fluorene=18.1% by weight.
Catalyst System and Prepolymerization Treatment
Before introducing it into the polymerization reactors, the solid catalyst component described above is contacted at 15xc2x0 C. for 30 minutes with aluminum triethyl (TEAL) in such quantity that the TEAL/Ti molar ratio be equal to 300.
The catalyst system is then subjected to prepolymerization by maintaining it in suspension in liquid propylene at 20xc2x0 C. for about 9 minutes before introducing it into the first polymerization reactor.